Bordetella pertussis produces several protein toxins including pertussis toxin and dermonecrotic toxin (heat labile toxin or HLT). HLT, injected subcutaneously into suckling mice, induces a lcoal pronounced hemorrhagic lesion. The extent, localization, and nature of this toxin-induced injury suggests HLT contributes to te pathogenesis of B. pertussis. Conditions for assay of the toxin using the suckling mouse model have been established. By use of conventional protein purification techniques and high pressure liquid chromatography, homogeneous preparations of HLT have been obtained. These studies show that the toxin is a single polypeptide chain with a molecular weight of about 150,000. The purified toxin has been subjected to amino acid analysis and has been used to produce antibodies. These antibodies have been used to document the purity of current toxin preparation. In addition, HLT antibodies neutralize toxic activity and high titer sera (1/20,000) have been obtained from immunized rabbits. HLT, in addition to mediating a dermonecrotic lesion, was shown to be lethal, induce a short term elevation (3 days) of white blood cells, and elicit and marked atrophy and "bleaching" of the spleen. The significance of these changes to possible biochemical mechanisms of action and their relation to the pathogenesis of pertussis are being investigated.